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| Planet Nine | |
|---|---|
| Name | Planet Nine |
| Caption | Artist's concept |
| Discoverer | Mike Brown and Konstantin Batygin |
| Discovery date | 2016 (hypothesis) |
| Semimajor axis | ~400–800 AU (estimated) |
| Eccentricity | ~0.2–0.6 (estimated) |
| Inclination | ~15–30° |
| Period | ~10,000–20,000 years (estimated) |
| Mass | ~5–10 M⊕ (estimated) |
Planet Nine is a hypothetical trans-Neptunian planet proposed to explain clustering in the orbits of distant solar system objects. The hypothesis was articulated by Mike Brown and Konstantin Batygin in 2016 following analyses of extreme trans-Neptunian objects discovered by surveys led by Palomar Observatory teams and others. The proposal has stimulated follow-up work across observational facilities such as ALMA, Subaru Telescope, and theoretical groups at institutions including Caltech and Harvard University.
The Planet Nine hypothesis arose from analysis of extreme trans-Neptunian objects (ETNOs) like Sedna (minor planet), 2012 VP113, and clustered objects discovered in surveys using Palomar Observatory and the Canada–France–Hawaii Telescope. In 2016 Mike Brown and Konstantin Batygin published papers interpreting orbital clustering similar to resonant behavior influenced by a distant perturber; contemporaneous work by researchers at Institute for Advanced Study and teams at University of Arizona refined the statistical arguments. Media coverage by outlets such as Nature (journal), Science (journal), and The New York Times popularized the idea, prompting observational programs at Keck Observatory and citizen science interest via projects like Zooniverse.
Modeling by teams at Caltech and collaborators predicts a perturber with mass in the super-Earth range (~5–10 Earth masses) on an eccentric, inclined orbit with semimajor axis estimates often between ~400 and ~800 AU and orbital periods of order 10,000–20,000 years. Simulations using N-body codes from groups at University of California, Berkeley and Princeton University explored ranges of eccentricity and inclination that produce the observed apsidal and nodal clustering of ETNOs such as 2015 TG387. Predicted observational signatures include slow apparent motion against background stars accessible to surveys using the Subaru Telescope Hyper Suprime-Cam, and infrared emission potentially detectable by instruments on Spitzer Space Telescope or JWST under certain albedo assumptions.
Support for the hypothesis rests on claimed alignments of arguments of perihelion and longitudes of ascending node among a sample of ETNOs including Sedna (minor planet), 2012 VP113, and 2014 SR349. Statistical analyses from teams at University of Oxford and University of Cambridge debated significance, with counteranalyses from researchers at Southwest Research Institute and Max Planck Institute for Solar System Research emphasizing survey selection biases documented by the Outer Solar System Origins Survey. Direct imaging limits from Pan-STARRS, WISE, and surveys using the Large Synoptic Survey Telescope (now Vera C. Rubin Observatory) constrain bright, nearby parameter space, while millimeter limits from ALMA and occultation studies by groups at Cornell University provide additional boundaries.
Proposed formation scenarios include in situ formation in an extended protoplanetary disk debated by modelers at Princeton University and University of Toronto, capture from a passing star cluster member during the Sun’s birth cluster as explored by researchers at Harvard University and University of Copenhagen, and scattering and outward migration driven by early interactions with giant planets modeled by groups at Southwest Research Institute and NASA Jet Propulsion Laboratory. Simulations invoking stellar encounters reference studies of the Sun’s birth environment by teams at Max Planck Institute for Astronomy and University of California, Santa Cruz. Alternative dynamical histories examine Kozai–Lidov cycles studied by researchers at Ohio State University and chaotic diffusion mechanisms from early solar system instability scenarios like the Nice model developed by groups at Observatoire de la Côte d'Azur and CNRS.
Targeted searches have been conducted by collaborations using facilities including Subaru Telescope, Keck Observatory, Palomar Observatory, Pan-STARRS, and the Vera C. Rubin Observatory planning teams. Space-based constraints leveraged data from WISE and proposal-driven follow-ups with Spitzer Space Telescope and JWST were discussed by teams at NASA centers and research universities. Citizen science and archival-mining efforts have involved groups at Zooniverse and projects hosted by University of Hawaii. Survey strategy papers from Caltech and University of Michigan emphasize cadence and sky coverage to overcome biases identified by the Outer Solar System Origins Survey and the Deep Ecliptic Survey.
Critiques emphasize observational bias and small-number statistics highlighted by researchers at University of Cambridge and Leiden University, as well as dynamical models showing clustering can arise from self-gravitating massive disks studied by teams at Institute for Advanced Study and Max Planck Institute for Astronomy. Alternative mechanisms include collective effects of a massive distant disk proposed in work at University of California, Santa Cruz and perturbations from passing stars examined by Harvard University groups. Some studies argue the clustering signal weakens with expanded surveys such as Outer Solar System Origins Survey and Pan-STARRS, a point underscored in critiques published in Monthly Notices of the Royal Astronomical Society and The Astrophysical Journal.
Category:Hypothetical planets